1. Field of the Invention
The invention concerns a method for determination of the position of a local coil on a patient bed in at least one spatial direction within a magnetic resonance tomograph apparatus. The invention moreover concerns a corresponding control device for a magnetic resonance system for determination of the position of a local coil in a scanner of a magnetic resonance system that implements such a method, a magnetic resonance system with such a control device as well as a corresponding computer program product that can be directly loaded into a processor of a programmable control device of a magnetic resonance system in order to implement such a method.
2. Description of the Prior Art
Modern magnetic resonance system normally operate with a number of various RF antennas (called coils in the following) for emission of radio-frequency pulses for exciting nuclei so as to emit magnetic resonance signals and/or for acquisition of the induced magnetic resonance signals. A magnetic resonance system normally has a larger whole-body coil (body coil) permanently installed in the magnetic resonance scanner. The whole-body coil is typically arranged cylindrically (for example with a structure known as a birdcage structure) around the patient acquisition chamber in which the patient is supported on a bed (often also called a patient positioning table) during the measurement. Furthermore, one or more small local coils or surface coils are frequently inserted into the magnetic resonance scanner. The local coils serve to acquire detailed images of body parts or organs of a patient that are located relatively near to the body surface. For this purpose the local coils are applied directly at the point of the patient at which the region to be examined is located. Given the use of such a local coil, in many cases RF energy is emitted with the whole-body coil (as a transmission coil) and the induced magnetic resonance signals are acquired with a local coil (as a reception coil).
For a magnetic resonance examination, it is important to know the exact position of the employed local coils relative to the bed (and thus relative to the patient). In principle, when the bed is mounted out of the scanner it is possible to manually measure the position of the coils using rulers, gauges, markings on the bed, etc. Such a measurement, however, is very complicated and additionally entails the risk that the measured positions are associated with the wrong coils, in particular given the usage of a number of coils, or given the usage of coil arrays composed of a number of coils. In principle, it is therefore simpler and safer to automatically determine the position of the local coils in the framework of a magnetic resonance measurement.
Such an automatic determination of the position of a local coil on a patient bed in a specific spatial direction can ensue, for example, according to a method described in DE 102 07 736 A1. A magnetic field gradient is applied in the appertaining spatial direction in which the position should be determined. A radio-frequency signal is then emitted with the whole-body coil or with a local coil, and an acquisition signal profile is measured along the magnetic field gradient by the appertaining local coil whose position is to be established. The position of the local coil in the appertaining spatial direction is determined with this signal profile.
A problem in the determination of the coil position according to this method occurs with regard to the essential determination of the support of the coil in the longitudinal direction of the bed (typically designated as the z-direction). The possibility exists that the coil is located entirely on the edge or outside of the scanner (and thus outside of the magnetic field) since the bed can be displaced (shifted) in the z-direction and is normally located only partially inside the scanner. If a local coil is located directly at the edge of the magnetic resonance scanner, the coil profile at the time varies unpredictably and chaotically. If the local coil is located outside of the scanner, a relevant coil profile can no longer be measured at all. This leads to the situation of a completely false position for such local coils being determined on occasion with conventional position determination methods.
These measurements therefore are normally supplemented by various quality criteria that are determined with the aid of additionally-acquired separate measurement values. Using these quality criteria, it can then be decided whether a measured signal profile represents a reasonable signal and thus can be used at all in a position determination algorithm in order to achieve a reliable result. This in turn reduces the number of the cases in which a result materializes at all. A number of measurements must additionally be implemented for determination of the coil position, which has an associated time loss.